High-throughput, low-latency next generation internet networks using optical label switching and high-speed optical header generation, detection and reinsertion

ABSTRACT

An optical signaling header technique applicable to optical networks wherein packet routing information is embedded in the same channel or wavelength as the data payload so that both the single-sideband modulated header and data payload propagate through network elements with the same path and the associated delays. The header routing information has sufficiently different characteristics from the data payload so that the signaling header can be detected without being affected by the data payload, and that the signaling header can also be removed without affecting the data payload. The signal routing technique can be overlaid onto the conventional network elements in a modular manner using two types of applique modules. The first type effects header encoding and decoding at the entry and exit points of the data payload into and out of the network; the second type effects header detection at each of the network elements.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/436,472, filedNov. 8, 1999, which is a continuation-in-part of application Ser. No.09/118,437 filed Jul. 17, 1998 now U.S. Pat. No. 6,111,673.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

This invention relates to optical communication systems and, moreparticularly, to an optical system which accommodates network trafficwith high throughput and low latency and effects high-speed headerdetection and generation.

2. Description of the Background Art

Recent research advances in optical Wavelength Division Multiplexing(WDM) technology have fostered the development of networks that areorders of magnitude higher in transmission bandwidth than existingcommercial networks. While such an increase in throughput is impressiveon its own, a corresponding decrease in network latency must also beachieved in order to realize the Next Generation Internet (NGI) visionof providing the next generation of ultra high speed networks that canmeet the requirements for supporting new applications, includingnational initiatives. Towards this end, current research efforts havefocused on developing an ultra-low latency Internet Protocol (IP) overWDM optical packet switching technology that promises to deliver thetwo-fold goal of both high throughput with low latency. Such efforts,while promising, have yet to fully realize this two-fold goal.

There are a number of challenging requirements in realizing such IP/WDMnetworks. First, the NGI network must inter-operate with the existingInternet and avoid protocol conflicts. Second, the NGI network mustprovide not only ultra low-latency, but must take advantage of bothpacket-switched (that is, bursty) IP traffic and circuit-switched WDMnetworks. Third, it is advantageous if the NGI network does not dependupon precise synchronization between signaling and data payload.Finally, a desired objective is that the NGI network accommodates datatraffic of various protocols and formats so that it is possible totransmit and receive IP as well as non-IP signals without the need forcomplicated synchronization or format conversion.

Comparison with Other Work

The Multi-Wavelength-Optical Network (MONET) system, as reported in thearticle “MONET: Multi-Wavelength Optical Networking” by R. E. Wagner, etal. and published in the Journal of Lightwave Technology, Vol. 14, No.6, June 1996, demonstrated a number of key milestones in opticalnetworks including transparent transmission of multi-wavelength throughmore than 12 reconfigurable network elements spread over the nationalscale fiber distance. The network, however, is circuit-switched andsuffers inefficiency in accommodating bursty traffic. The typicalconnection setup time from request to switching is a few seconds,limited by capabilities of both Network Control & Management (NC&M) andhardware. Recent efforts within the MONET program to improve on theefficiency concentrated on the “Just-in-Time signaling” scheme. Thismethod utilizes embedded 1510 nm NC&M signaling which precedes the datapayload by an estimated delay time. This estimation must be accuratelymade for each network configuration for every wavelength in order tosynchronize the signaling header and switching of the payload.

In accordance with the present invention, the optical packet header iscarried over the same wavelength as the packet payload data. Thisapproach mitigates the issue of header and payload synchronization.Furthermore, with a suitable use of optical delay at each intermediateoptical switch, it eliminates the need to estimate the initial burstdelay by incorporating the optical delay directly at the local switches.This makes a striking difference with Just-In-Time signaling in whichthe delay at each switch along the path needs to be known ahead of timeand must be entered in the calculation for the total delay. Lastly,there is little time wasted in requesting a connection time and actuallyachieving a connection. In comparison to a few second delays seen inMONET, the present inventive subject matter reduces the delay tominimal, only limited by the actual hardware switching delays at eachswitch. The current switching technology realizes delays of only severalmicroseconds, and shorter delays will be possible in the future. Such ashort delay can be incorporated by using an optical fiber delay line ateach network element utilizing switches. The present inventive subjectmatter achieves the lowest possible latency down to the fundamentallimit of the hardware, and no lower latency can be achieved by any othertechnique.

The Optical Networks Technology Consortium (ONTC) results were reportedin the article “Multiwavelength Reconfigurable WDM/ATM/SONET NetworkTestbed” by Chang et al. and published in the Journal of LightwaveTechnology, Vol. 14, No. 6, June 1996. Both Phase I (155 Mb/s,4-wavelength) and Phase II (2.5 Gb/s, 8-wavelength) of the ONTC programwere configured on a Multihop ATM-based network. While such an ATM basedarchitecture added a large overhead and excluded the possibility of asingle-hop network, the packet/header signaling was made possible byutilizing the isochronous ATM cell itself. This communication of NC&Minformation is made through the same optical wavelength, potentiallyoffering similar benefits as with the technique of the presentinvention. However, the inventive technique offers a number ofsignificant advantages over the ATM-based signaling. First, theinventive technique offers a single hop connection for the payloadwithout the need to convert to electrical signals and buffer thepackets. Second, it offers far more efficient utilization of thebandwidth by eliminating excessive overheads. Third, it allows strictlytransparent and ultra-low latency connections.

The DARPA sponsored All-Optical-Network (AON) Consortium results werereported in an article entitled “A Wideband All-Optical WDM Network” ,by I. P. Kaminow et al. and published in the IEEE Journal on SelectedAreas of Communication, Vol. 14, No. 5, June, 1996. There were actuallytwo parts of the AON program: WDM as reported in the aforementionedarticle, and TDM reported in a companion paper in the same issue. Firstthe WDM part of the AON program is first discussed, followed by the TDMpart.

The AON architecture is a three-level hierarchy of subnetworks, andresembles that of LANs, MANs, and WANs seen in computer networks. TheAON provides three basic services between Optical Terminals (OTs): A, B,and C services. A is a transparent circuit-switched service, B is atransparent time-scheduled TDM WDM service, and C is a non-transparentdatagram service used for signaling. The B service uses a structurewhere a 250 microsecond frame is used with 128 slots per frame. Within aslot or group of slots, a user is free to choose the modulation rate andformat. The B-service implemented on the AON architecture is closest tothe IP over WDM, which is the subject matter of the present invention.However, the separation of NC&M signaling in the C-service with thepayload in the B-service requires careful synchronization between thesignaling header and the payload. This requirement becomes far morestringent as the 250 microsecond frame is used with 128 slots per framewith arbitrary bit rates. Not only the synchronization has to occur atthe bit level, but this synchronization has to be achieved across theentire network. The scalability and interoperability are extremelydifficult since these do not go in steps with the networksynchronization requirement. The present inventive subject matterrequires only that the payload and the header are transmitted andreceived simultaneously, inter-operates with existing IP and non-IPtraffic, and offers scalability.

TDM efforts are aimed at 100 Gb/s bit rates. In principle, suchultrafast TDM networks have the potential to provide truly flexiblebandwidth on demand at burst rates of 100 Gb/s. However, there aresignificant technological challenges behind such high bit rate systemsmainly related to nonlinearities, dispersion, and polarizationdegradations in the fiber. While the soliton technologies can alleviatesome of the difficulties, it still requires extremely accuratesynchronization of the network—down to a few picoseconds. In addition,the header and the payload must have the identical bit rates, and as aconsequence, bit-rate transparent services are difficult to provide. Thesubject matter in accordance with the present invention does not dependon precise synchronization, relies on no 100 Gb/s technologies, andoffers transparent services.

The Cisco Corporation recently announced a product based onTag-Switching and the general description of Cisco's Tag-Switching isavailable at the world-wide-web site,(http://www.cisco.com/warp/public/732/tag/). Cisco's (electronic) TagSwitching assigns a label or “tag” to packets traversing a network ofrouters and switches. In a conventional router network, each packet mustbe processed by each router to determine the next hop of the packettoward its final destination. In an (electronic) Tag Switching network,tags are assigned to destination networks or hosts. Packets then areswitched through the network with each node simply swapping tags ratherthan processing each packet. An (electronic) Tag Switching network willconsist of a core of (electronic) tag switches (either conventionalrouters or switches), which connect to (electronic) tag edge routers onthe network's periphery. (Electronic) Tag edge routers and tag switchesuse standard routing protocols to identify routes through the network.These systems then use the tables generated by the routing protocols toassign and distribute tag information via a Tag Distribution Protocol.Tag switches and tag edge routers receive the Tag Distribution Protocolinformation and build a forwarding database. The database mapsparticular destinations to the tags associated with those destinationsand the ports through which they are reachable.

When a tag edge router receives a packet for forwarding across the tagnetwork, it analyzes the network-layer header and performs applicablenetwork layer services. It then selects a route for the packet from itsrouting tables, applies a tag and forwards the packet to the next-hoptag switch.

The tag switch receives the tagged packet and switches the packet basedsolely on the tag, without re-analyzing the network-layer header. Thepacket reaches the tag edge router at the egress point of the network,where the tag is stripped off and the packet delivered. After Cisco madeits announcement about (Electronic) Tag Switching, the IETF (InternetEngineering Task Force) has recommended a MPLS (Multi-protocol LabelSwitching) to implement standardized, vendor-neutral (electronic)tag-switching function in routers and switches, including ATM switches.

A number of features in the Cisco's (electronic) Tag Switching issimilar to the Optical Tag Switching which is the subject matter of thepresent invention, with the features aimed at the similar goals ofsimplifying the processing required for packet routing. The keydifferences are as follows. First, the optical tag switching is purelyoptical in the sense that both tag and data payload are in an opticalform. While each plug-and-play module (a component of the presentinventive system) senses the optical tag, the actual packet does notundergo optical-to-electrical conversion until it comes out of thenetwork. The Cisco's (electronic) Tag Switching will be all electrical,and applies electronic detection, processing, and retransmission to eachpacket at each router. Secondly, the Optical Tag Switching of thepresent invention achieves lowest possible latency and does not rely onutilizing buffers. Electronic tag switching will have far greaterlatency due to electronic processing and electronic buffering. Thirdly,the Optical Tag Switching of the present invention utilizes pathdeflection and/or wavelength conversion to resolve blocking due tocontention of the packets, whereas the Electronic Tag Switching willonly utilize electronic buffering as a means to achieve contentionresolution at the cost of increased latency, and the performance isstrongly dependent on packet size. The present invention covers packetsof any length. Lastly, the Optical Tag Switching of the presentinvention achieves a strictly transparent network in which data of anyformat and protocol can be routed so long as it has a proper opticaltag. Hence the data can be digital of any bit rate and modulationformats. The Electronic Tag Switching requires that data payload to havethe given digital bit rate identical to the electronic tag since therouters must buffer them electronically.

Another representative technology that serves as background to thepresent invention is the so-called Session Deflection Virtual CircuitProtocol (SDVC), which is based on a deflection routing method. Thepaper entitled “The Manhattan Street Network”, by N. F. Maxemchuk” aspublished in the Proceedings on IEEE Globecom '85, pp 255-261, December1985, discusses that when two packets attempt to go to the samedestination, one packet can be randomly chosen for the preferred outputlink and the other packet is “deflected” to the non-preferred link. Thismeans that packets will occasionally take paths that are not shortestpaths. The deflection method utilized by the present invention does not‘randomly’ select the packet to go to the most preferred path; rather,it attempts to look into the priorities of the packets, and send thehigher priority packet to be routed to the preferred path. The packetswill be deflected if they have lower priorities; however, both ‘pathdeflection’ and ‘wavelength deflection’ are utilized. The pathdeflection is similar to conventional SDVC in that the optical packetwill be simply routed to the path of the next preference at the samewavelength. The wavelength deflection allows the optical packet to berouted to the most preferred path but at a different wavelength. Thiswavelength deflection is achieved by wavelength conversion at thenetwork elements. Partially limited wavelength conversion is utilized,meaning not all wavelengths will be available as destination wavelengthsfor a given originating wavelength. The wavelength deflection allowsresolution of blocking due to wavelength contentions without increasingthe path delay. The combination of path and wavelength deflectionsoffers sufficiently large additional connectivities for resolving packetcontentions; however, the degree of partial wavelength conversion can beincreased when the blocking rate starts to rise. Such scalability andflexibility of the network are not addressed by conventional SDVC.

Besides the foregoing overall system considerations elucidated above,there is also the issue of how to effectively detect and/or re-insert aheader which is combined with a data payload for propagation over thenetwork using the same optical wavelength. The primary focus in theliterature has been on a technique for combining sub-carrier headerswith a baseband data payload. The very first two articles addressingthis issued were published in 1992 by A. Bidman et. al., who combined a2.56 Gb/s data payload with a 40 Mb/s header on 3 GHz carrier [A.Budman, E. Eichen, J. Schalafer, R. Olshansky, and F. McAleavey,“Multigigabit optical packet switch for self-routing networks withsubcarrier addressing,” Techical Digest, paper TuO4, pp.90-91, OFC'92],and W. I. Way et al., who combined a 2.488 Gb/s data payload with atunable microwave pilot tone (tuned between 2.520 and 2.690 GHz) toroute SONET packet in a WDM ring network via acousto-optical tunablefilter [W. I. Way, D. A. Smith, J. J. Johnson, H. Izadpanah, and H.Johnson, “Self-routing WDM high-capacity SONET ring network,” TechnicalDigest, paper TuO2, pp.86-87, OFC'92, and W. I. Way, D. A. Smith, J. J.Johnson, and H. Izadpanah, “A self-routing WDM high-capacity SONET ringnetwork,” IEEE Photonics Technology Letters, vol.4, pp.402-404, April1992.2,3]. Both of these articles used a single laser diode to carry thedata payload and sub-carrier header. This technique has also beenextensively studied in a local-area DWDM optical packet-switched network[R. T. Hofmeister, L. G. Katzovsky, C. L. Lu, P. Poggiolini, and F.Yang, “CORD: optical packet-switched network testbed,” Fiber andIntegrated Optics, vol.16, pp.199-219, 1997], and several otherall-optical networks [E. Park and A. E. Willner, “Network demonstrationof self-routing wavelength packets using an all-optical wavelengthshifter and QPSK subcarrier routing control,” IEEE Photonics TechnologyLetters, vol.8, pp.938-940, 1996; and M. Shell, M. Vaughn, A. Wang, D.J. Blumenthal, P. J. Rigole, and S. Nilsson, “Experimental demonstrationof an all-optical routing node for multihop wavelength routed networks,”IEEE Photonic Technology Letters, vol.8, pp.1391-1393, 1996].

Instead of combing a sub-carrier header with the data payload in theelectrical domain, they have also been combined in the optical domain byusing two laser diodes at different wavelengths [B. H. Wang, K. Y. Yen,and W. I. Way, “Demonstration of gigabit WDMA networks usingparallel-processed sub-carrier hopping pilot-tone (p³) signalingtechnique,” IEEE Photonics Technology Letters, vol.8, pp.933-934, July1996].

However, using two wavelengths to transport data payload and headerseparately may not be practical in the following sense: in anall-optical DWDM network, it is preferred that the header, which maycontain network operations information, travels along the same routes asdata payload so that it can truthfully report the updated status of thedata payload. If the header and the data payload were carried bydifferent wavelengths, they could be routed in the network with entirelydifferent paths, and the header may not report what the data payload hasreally experienced. Therefore, although it is preferred that thesub-carrier header and the data payload be carried by the samewavelength, the art is devoid of such teachings and suggestions.

The sub-carrier pilot-tone concept presented in Wang et al. was extendedto multiple pilot tones by Shieh et al. [W. Shieh and A. E. Willner, “Awavelength routing node using multifunctional semiconductor opticalamplifiers and multiple-pilot-tone-coded subcarrier control headers,”IEEE Photonics Technology Letters, vol.9, pp.1268-1270, September1997.], mainly for the purpose of increasing the number of networkaddresses.

Recently, consideration has been given to ‘header replacement’ for thehigh-throughput operation in a packet-switched network in which datapaths change due to link outages, output-port contention, and variabletraffic patterns. Moreover, header replacement could be useful formaintaining protocol compatibility at gateways between differentnetworks. However, the only method which has been reported is fortime-division-multiplexed header and data payload requires an extremelyhigh accuracy of timing synchronization among network nodes [X. Jiang,X. P. Chen, and A. E. Willner, “All optical wavelength independentpacket header replacement using a long CW region generated directly fromthe packet flag,” IEEE Photonics Technology Letters, vol.9,pp.1638-1640, November 1998].

Most recently, Blumenthal et al., in an article entitled “WDM OpticalTag Switching with Packet-Rate Wavelength Conversion and SubcarrierMultiplexed Addressing”, OFC 1999, Conference Digest, pages 162-164,report experimental results of all-optical IP tag switching for WDMswitched networks. However, the experimental system is a non-burstsystem and, moreover, no propagation of the resultant signal over actualfiber is discussed. It is anticipated that the propagation distance willbe substantially limited whenever the system is deployed with opticalfiber because of phase dispersion effects in the optical fiber.

From this overview of the art pertaining to details of header generationand detection, it is readily understood that the art is devoid ofteachings and suggestions wherein sub-carrier multiplexed packet datapayload and multiple sub-carrier headers (including old and new ones)are deployed so that a >2.5 Gbps IP packet can be routed through anational all-optical DWDM network by the (successive) guidance of thesesub-carrier headers, with the total number of sub-carrier headers thatcan be written is in the range of forty or more.

SUMMARY OF THE INVENTION

The present invention utilizes a unique optical signaling headertechnique applicable to optical networks. Packet routing information isembedded in the same channel or wavelength as the data payload so thatboth the header and data information propagate through the network withthe same path and the associated delays. However, the header routinginformation has sufficiently different characteristics from the datapayload so that the signaling header can be detected without beingaffected by the data payload and that the signaling header can also bestripped off without affecting the data payload. The inventive subjectmatter allows such a unique signal routing method to be overlaid ontothe conventional network elements in a modular manner, including theinsertion, detection and processing of the optical header.

In accordance with one broad method aspect of the present inventioncommensurate with the overall NGI system, a method for propagating adata payload from an input network element to an output network elementin a wavelength division multiplexing system composed of a plurality ofnetwork elements, given that the data payload has a given format andprotocol, includes the following steps: (a) adding a single-sidebandheader to the data payload prior to inputting the data payload to theinput network element to produce an optical signal, the header having aformat and protocol and being indicative of the local route through eachof the network elements for the data payload and the header, the formatand protocol of the data payload being independent of the format andprotocol of the header; and (b) detecting the header at each of thenetwork elements as the data payload and header propagate through theWDM network, wherein the header is conveyed by a distinct carrierfrequency such that the single-sideband spectrum of the header occupiesa frequency band above the data payload, such that the step of detectingincludes (i) opto-electrically converting the optical signal to detectthe header, (ii) processing the header to produce a switch controlsignal to route the incoming optical signal, (iii) optically filteringthe optical signal with a reflective part of a notch filter to deletethe header and recover the data payload, and (iv) inserting a newsingle-sideband header at the given frequency band into the opticalsignal in place of the deleted header.

In accordance with a broad system aspect of the present inventioncommensurate with the overall NGI system, a system for propagating adata payload from an input network element to an output network elementin a wavelength division multiplexing system composed of a plurality ofnetwork elements, given that the data payload has a given format andprotocol, includes: (a) an adder for adding a single-sideband header tothe data payload prior to inputting the data payload to the inputnetwork element to produce an optical signal, the header having a formatand protocol and being indicative of the local route through each of thenetwork elements for the data payload and the header, the format andprotocol of the data payload being independent of the format andprotocol of the header; and (b) a detector for detecting the header ateach of the network elements as the data payload and header propagatethrough the WDM network, wherein the header is conveyed by a distinctcarrier frequency such that the single-sideband spectrum of the headeroccupies a frequency band above the data payload, such that systemfurther includes (i) an opto-electrical converter for detecting theoptical signal to produce the header, (ii) a processor for processingthe detected header to produce a switch control signal to route theincoming optical signal, (iii) an optical filter for filtering theoptical signal with a reflective part of the notch filter to delete theheader and recover the data payload, and (iv) means for inserting a newsingle-sideband header at the given frequency band into the opticalsignal in place of the deleted header.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a pictorial representation of a general network illustratingthe coupling between the optical and electrical layers of the network;

FIG. 2 illustrates the optical layer of the network of FIG. 1 showingthe relationship between the optical signal header and data payload, andthe use of the header/payload in network setup;

FIG. 3 is a high-level block diagram an optical transmitter inaccordance with the present invention for header encoding;

FIG. 4 is a high-level block diagram an optical receiver in accordancewith the present invention for header decoding;

FIG. 5 is illustrative of a WDM circuit-switched backbone network;

FIG. 6 illustrates a network element of FIG. 1 with its embedded switchand the use of local routing tables;

FIG. 7 depicts a block diagram of an illustrative embodiment of a headerencoder circuit for the Plug-&-Play module of FIG. 3;

FIG. 8 depicts a block diagram of an illustrative embodiment of a headerremover circuit for the Plug-&-Play module of FIG. 3;

FIG. 9 depicts a block diagram of an illustrative embodiment of a headerdetector circuit for the Plug-&-Play module of FIG. 4;

FIG. 10 depicts a block diagram for a more detailed embodiment of FIG. 4wherein the label-switch controller includes interposed demultiplexers,and header detectors and fast memory;

FIG. 11 is a flow diagram for the processing effected by eachlabel-switch controller of FIG. 10;

FIG. 12 is a block diagram of circuitry for removing the header with thereflective part of a notch filter and for inserting a newsingle-sideband modulated header; and

FIG. 13 is a block diagram of circuitry for the detecting the headerwith the transmission part of the notch filter, for removing the headerwith the reflective part of a notch filter, and for inserting a newsingle-sideband modulated header.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

In order to gain an insight into the fundamental principles inaccordance with the present invention as well as to introduceterminology useful in the sequel, an overview is first presented,followed by an elucidation of an illustrative embodiment.

Overview

The present invention relates to a network for realizing low latency,high throughput, and cost-effective bandwidth-on-demand for large blocksof data for NGI applications. Cost-effective and interoperable upgradesto the network are realized by interposing portable ‘Plug-and-Play’modules on the existing WDM network elements to effect so-called “WDMoptical label switching” or, synonymously, “optical label switching”.The invention impacts primarily the hardware for the NGI network fromthe network element design perspective.

As alluded to, the methodology carried out by the network andconcomitant circuitry for implementing the network are engendered by atechnique called WDM optical label-switching—defined as the dynamicgeneration of a routing path for a burst duration by an in-band opticalsignaling header. Data packets are routed through the WDM network usingan in-band WDM signaling header for each packet. At a switching node,the signaling header is processed and the header and the data payload(1) may be immediately forwarded through an already existing flow stateconnection, or (2) a path can be setup for a burst duration to handlethe header and the data payload. WDM label-switching enables highlyefficient routing and throughput, and reduces the number of IP-levelhops required by keeping the packets routing at the optical level to onehop as managed by the Network Control and Management (NC&M) whichcreates and maintains routing information.

The depiction of FIG. 1 shows the inter-relation between optical layer120 and electrical layer 110 of generic network 100 as provided byintermediate layer 130 coupling the optical layer and the electricallayer. Electrical layer 110 is shown, for simplicity, as being composedof two conventional IP routers 111 and 112. Optical layer 120 is shownas being composed of network elements or nodes 121-125. Intermediatelayer 130 depicts conventional ATM/SONET system 131 coupling IP router112 to network element 122. Also shown as part of layer 130 is headernetwork 132, which in accordance with the present invention, couples IProuter 111 to network element 121. FIG. 1 pictorially illustrates thelocation of network 132 on a national-scale, transparent VDM-basedbackbone network with full interoperability and reconfigurability. It isimportant to emphasize at this point that the elements of FIG. 1 areillustrative of one embodiment in accordance with the present invention;thus, for example, element 111 may, in another embodiment, be an ATMrouter or even a switch.

Now with reference to FIG. 2, optical layer 120 of FIG. 1 is shown inmore detail including the basic technique, in accordance with thepresent invention, for setting up a fast connection in optical network201, composed of network elements 121-125; the setup uses opticalsignaling header 210 for the accompanying data payload 211. Thistechnique combines the advantages of circuit-switched based WDM andpacket-switched based IP technologies. New signaling information isadded in the form of an optical signal header 210, which is carriedin-band within each wavelength in the multi-wavelength transportenvironment. Optical signaling header 210 is a label containing routingand control information such as the source, destination, priority, andthe length of the packet, and propagates through optical network 201preceding data payload 211. Each WDM network element 121-125 sensesoptical signaling header 210, looks-up a connection table (discussedlater), and takes necessary steps such as cross-connections, add, drop,or drop-and-continue. The connection table is constantly updated bycontinuous communication between NC&M 220 and WDM network elements121-125 through logical connections, such as channel 221. Data payload211, which follows optical signaling header 210, is routed through apath in each network element (discussed later) as established by theconnection. With the arrangement of FIG. 2, there is no need to managethe time delay between optical signaling header 210 and data payload211, shown by T in FIG. 2, because each network element provides theoptical delay needed for the short time required for connection set-upwithin each network element via delay on an interposed fiber. Moreover,the format and protocol of the data payload is independent of that ofthe header, that is, for a given network whereas the format and protocolof the header are pre-determined, the format and the protocol of thedata payload can be the same as or different from those of the header.

Each destination is associated with a preferred path which wouldminimize ‘the cost’—in FIG. 2, the overall path from source 123 todestination 122 includes paths 201 and 202 in cascade, both utilizingwavelength WP. This cost is computed based on the total propagationdistance, the number of hops, and the traffic load. The preferredwavelength is defaulted to the original wavelength. For example, thepreferred wavelength on path 202 is WP. If this preferred path at thedefault wavelength is already occupied by another packet, then networkelement 121 quickly decides if there is an available alternatewavelength WA through the same preferred path. This alternate wavelengthmust be one of the choices offered by the limited wavelength conversionin network element 121. If there is no choice of wavelengths whichallows transport of the packet through the most preferred path, the nextpreferred path is selected (path deflection). For example, in FIG. 2,paths 203 and 204 in cascade may represent the alternative path. At thispoint, the preferred wavelength will default back to the originalwavelength WP. The identical process of looking for an alternatewavelength can proceed if this default wavelength is again alreadyoccupied. In FIG. 2, path 203 is an alternative path with the samewavelength WP, and path 204 is an alternate path using alternatewavelength WA. In an unlikely case where there is no combination of pathand wavelength deflection that can offer transport of the packet,network element 121 will decide to drop the packet of lower priority. Inother words, the new packet transport through the preferred path at theoriginating wavelength takes place by dropping the other packet of thelower priority which is already occupying the preferred path.

Network elements 121-125 are augmented with two types of so-called‘Plug-and-Play’ modules to efficiently handle bursty traffic byproviding packet switching capabilities to conventional circuit-switchedWDM network elements 121-125 whereby signaling headers are encoded ontoIP packets and are removed when necessary.

The first type of ‘Plug-and-Play’ module, represented by electro-opticalelement 132 of FIG. 1, is now shown in block diagram form in FIG. 3.Whereas conceptually module 132 is a stand-alone element, in practice,module 132 is integrated with network element 121 as is shown in FIG. 3;module 132 is interposed between compliant client interface (CCI) 310 ofnetwork element 121 and IP router 111 to encode optical signaling header210 onto the packets added into the network via header encoder 321, andto remove optical signaling header 210 from the packets dropping out ofthe network via header remover 322.

Generally, encoding/removing module 132 is placed where the IP trafficis interfaced into and out of the WDM network, which is between theclient interface of the network element and the IP routers. The clientinterfaces can be either a CCI-type or a non-compliant client interfaces(NCI)-type. At these interfaces, header encoder 321 puts optical header210 carrying the destination and other information in front of datapayload 211 as the IP signal is transported into network 201. Opticalheader 210 is based on the IP signal's original IP address, which isobtained from IP router 111 through interface 311, and is encoded in theoptical domain by an optical modulator (discussed later). Signalingheader remover 322 deletes header 210 from the optical signal droppedvia a client interface, and provides an electrical IP packet to IProuter 111.

More specifically, module 132 accepts the electrical signal from IProuter 111, converts the electrical signal to a desired compliantwavelength optical signal, and places optical header 210 in front of theentire packet. Module 132 communicates with NC&M 220 and buffers thedata before optically converting the data if requested by NC&M 220.Module 132 employs an optical transmitter (discussed later) with thewavelength matched to the client interface wavelength. (As indicatedlater but instructive to mention here, module 132 is also compatiblewith NCI 404 of FIG. 4 since the wavelength adaptation occurs in theNCI; however, the bit-rate-compatibility of NCI wavelength adaption andthe IP signal with optical headers must be established in advance.)

FIG. 4 depicts a second type of ‘Plug-and-Play’ module, optical element410, which is associated with each WDM network element 121-125, sayelement 121 for discussion purposes. Module 410 is interposed betweenconventional network element circuit switch controller 420 andconventional switching device 430. Module 410 detects information fromeach signaling header 210 propagating over any fiber 401-403, asprovided to module 410 by tapped fiber paths 404-406. Module 410functions to achieve very rapid table look-up and fast signaling toswitching device 430. Switch controller 420 is functionally equivalentto the conventional “craft interface” used for controlling the networkelements; however, in this case, the purpose of this switch controller420 is to accept the circuit-switched signaling from NC&M 220 anddetermine which control commands are to be sent to label switchcontroller 410 based on the priority. Thus, label switch controller 410receives circuit-switched control signals from network element circuitswitch controller 420, as well as information as derived from eachsignaling header 210, and intelligently chooses between thecircuit-switched and the label-switched control schemes. The switches(discussed later) comprising switching device 430 also achieve rapidswitching. The delay imposed by fibers 415, 416, or 417, which areplaced in input paths 401-403 to switching device 430, are such that thedelay is larger than the total time it takes to read signaling header210, to complete a table look-up, and to effect switching.Approximately, a 2 km fiber provides 10 microsecond processing time. Thetypes of WDM network elements represented by elements 121-125 and whichencompass switching device 430 include: Wavelength Add-Drop Multiplexers(WADMs); Wavelength Selective Crossconnects (WSXCs); and WavelengthInterchanging Crossconnects (WIXCs) with limited wavelength conversioncapabilities.

In operation, module 410 taps a small fraction of the optical signalsappearing on paths 401-403 in order to detect information in eachsignaling header 210, and determine the appropriate commands forswitching device 430 after looking up the connection table stored inmodule 410. The fiber delay is placed in paths 401-403 so that thepacket having header 210 and payload 211 reaches switching device 430only after the actual switching occurs. This fiber delay is specific tothe delay associated with header detection, table look-up, andswitching, and can typically be accomplished in about 10 microsecondswith about 2 km fiber delay in fibers 415-417.

Packets are routed through network 201 using the information insignaling header 210 of each packet. When a packet arrives at a networkelement, signaling header 210 is read and either the packet (a) isrouted to a new appropriate outbound port chosen according to the labelrouting look-up table, or (b) is immediately forwarded through analready existing label-switching originated connection within thenetwork element. The latter case is referred to as “flow switching” andis supported as part of optical label-switching; flow switching is usedfor large volume bursty mode traffic.

Label-switched routing look-up tables are included in network elements121-125 in order to rapidly route the optical packet through the networkelement whenever a flow switching state is not set-up. The connectionset-up request conveyed by optical signaling header 210 is rapidlycompared against the label-switch routing look-up table within eachnetwork element. In some cases, the optimal connections for the mostefficient signal routing may already be occupied. The possibleconnection look up table is also configured to already provide analternate wavelength assignment or an alternate path to route thesignal. Providing a limited number of (at least one) alternativewavelength significantly reduces the blocking probability. Thealternative wavelength routing also achieves the same propagation delayand number of hops as the optimal case, and eliminates the difficultiesin sequencing multiple packets. The alternate path routing canpotentially increase the delay and the number of hops, and the signal-tonoise-ratio of the packets are optically monitored to eliminate anypossibility of packets being routed through a large number of hops. Inthe case where a second path or wavelength is not available, contentionat an outbound link can be settled on a first-come, first-serve basis oron a priority basis. The information is presented to a regular IP routerand then is reviewed by higher layer protocols, using retransmissionwhen necessary.

ROUTING EXAMPLE

An illustrative WDM circuit-switched backbone network 500 forcommunicating packets among end-users in certain large cities in theUnited States is shown in pictorial form in FIG. 5—network 500 is firstdiscussed in terms of its conventional operation, that is, before theoverlay of WDM optical label switching in accordance with the presentinvention is presented.

With reference to FIG. 5, it is supposed that New York City is served bynetwork element 501, Chicago is served by network element 502, . . . ,Los Angeles is served by network element 504, . . . , and Minneapolis bynetwork element 507. (Network elements may also be referred to as nodesin the sequel.) Moreover, NC&M 220 has logical connections (shown bydashed lines, such as channel 221 to network element 501 and channel 222to network element 507) to all network elements 501-507 via physicallayer optical supervisory channels; there is continuous communicationamong NC&M 220 and network elements 501-507. NC&M 220 periodicallyrequests and receives information about: (a) the general state of eachnetwork element (e.g., whether it is operational or shut down for anemergency); (b) the optical wavelengths provided by each network element(e.g., network element 501 is shown as being served by optical fibermedium 531 having wavelength W1 and optical fiber medium 532 havingwavelength W2 which connect to network elements 502 (Chicago) and 505(Boston), respectively); and (c) the ports which are served by thewavelengths (e.g., port 510 of element 501 is associated with anincoming client interface conveying packet 520, port 511 is associatedwith W1 and port 512 is associated with W2, whereas port 513 of element502 is associated with W2).

Thus, NC&M 220 has stored at any instant the global informationnecessary to formulate routes to carry the incoming packet traffic bythe network elements. Accordingly, periodically NC&M 220 determines therouting information in the form of, for example, global routing tables,and downloads the global routing t tables to each of the elements usingsupervisory channels 221, 222, . . . . The global routing tablesconfigure the ports of the network elements to create certaincommunication links. For example, NC&M 220 may determine, based upontraffic demand and statistics, that a fiber optic link from New YorkCity to Los Angeles (network elements 501 and 504, respectively) ispresently required, and the link will be composed, in series, of: W1coupling port 511 of element 501 to port 513 in network element 502; W1coupling port 514 of element 502 to port 515 of element 503; and W2coupling port 516 of element 503 to port 517 of element 504. Then, inputpacket 520 incoming to network element 501 (New York City) and having adestination of network element 504 (Los Angeles) is immediately routedover this established link. At network element 504, the propagatedpacket is delivered as output packet 521 via client interface port 518.

In a similar manner, a dedicated path between elements 506 and 507 (St.Louis and Minneapolis, respectively) is shown as established using W3between network elements 506 and 502, and W2 between elements 502 and507.

Links generated in this manner—as based up on the global routingtables—are characterized by their rigidity, that is, it takes severalseconds for NC&M 220 to determine the connections to establish thelinks, to download the connectivity information for the links, andestablish the in put and output ports for each network element. Eachlink has characteristics of a circuit-switched connection, that is, itis basically a permanent connection or a dedicated path or “pipe” forlong intervals, and only NC&M 220 can tear down and re-establish a linkin normal operation. The benefit of such a dedicated path is thattraffic having an origin and a destination that maps into analready-established dedicated path can be immediately routed without theneed for any set-up. On the other hand, the dedicated path can be, andmost often is, inefficient in the sense that the dedicated path may beonly used a small percentage of the time (e.g., 20%-50% over the set-upperiod). Moreover, switching device 430 (see FIG. 4), embedded in eachnetwork element which interconnects input and output ports, has only afinite number of input/output ports. If the above scenario is changed sothat link from St. Louis to Minneapolis is required and a port alreadyassigned to the New York to Los Angeles link is to be used (e.g., port514 of network element 502), then there is a time delay until NC&M 220can respond and alter the global routing tables accordingly.

Now the example is expanded so that the subject matter in accordancewith the principles of the present invention is overlaid on the abovedescription. First, a parameter called the “label-switched state” isintroduced and its use in routing is discussed; then, in the nextparagraph, the manner of generating the label-switch state iselucidated. The label-switch state engenders optical label switching.

NC&M 220 is further arranged so that it may assign the label-switchstate to each packet incoming to a network element from a clientinterface—the label-switch state is appended by Plug & Play module 132and, for the purposes of the present discussion, the label-switch stateis commensurate with header 210 (see FIG. 2). The label-switch state iscomputed by NC&M 220 and downloaded to each network element 501-507 inthe form of a local routing table. With reference to FIG. 6, there isshown network element 501 and its embedded switch 601 in pictorial form.Also shown is incoming optical fiber 602, with delay loop 603, carryingpacket 620 composed of header 210 and payload 211—payload 211 in thiscase is packet 520 from FIG. 5. Fiber 6022 delivers a delayed version ofpacket 620 to network element 501. Also, a portion of the light energyappearing on fiber 602 is tapped via fiber 6021 and inputted to opticalmodule 410 which processes the incoming packet 620 to detect header210—header 210 for packet 620 is shown as being composed of thelabel-switch state ‘11101011000’, identified by reference numeral 615.Also shown in FIG. 6 is local look-up table 610, being composed of twocolumns, namely, “Label-switch State” (column 611), and “Local Address”(column 612). The particular label-switch state for packet 620 iscross-referenced in look-up table 610 to determine the routing of theincoming packet. In this case, the label-switch state for packet 620 isthe entry in the fourth row of look-up table 610. The local switchaddress corresponding to this label-switch state is “0111”, which isinterpreted as follows: the first two binary digits indicate theincoming port, and the second two binary digits indicate the outputport. In this case, for the exemplary four-input, four-output switch,the incoming packet is to be routed from input port “01” to output port“11”, so switch 601 is switched accordingly (as shown). After the delayprovided by fiber delay 603, the incoming packet on fiber 6022 ispropagated onto fiber 604 via switch 601.

The foregoing description of label-switch state indicates how it isused. The manner of generating the label-switch state is now considered.NC&M 220, again on a periodic basis, compiles a set of local look-uptables for routing/switching the packet through each correspondingnetwork element (such as table 610 for network element 501), and eachlook-up table is then downloaded to the corresponding network element.The generation of each look-up table takes into account NC&M 220'sglobal knowledge of the network 500. For instance, if incoming packet620 to network 501 is destined for network 504 (again, New York to LosAngeles), if port 510 is associated with incoming port “01” and servesfiber 602, and if outgoing port 511 is associated with outgoing port“11” and serves fiber 604, then NC&M 220 is able to generate theappropriate entry in look-up table 610 (namely, the fourth row) anddownload table 610 to network element 510. Now, when packet 520 isprocessed by electro-optical module 132 so as to add header 210 topacket 520 to create augmented packet 620, NC&M 220's knowledge of thedownloaded local routing tables as well as the knowledge of thedestination address embedded in packet 520 as obtained via module 132enables NC&M 220 to instruct module 132 to add the appropriatelabel-switch state as header 210—in this case ‘11101011000’.

It can be readily appreciated that processing a packet using thelabel-switch state parameter is bursty in nature, that is, after switch601 is set-up to handle the incoming label-switch state, switch 601 maybe returned to its state prior to processing the flow state. Forexample, switch 601 may have interconnected input port ‘01’ to outputport ‘10’ prior to the arrival of packet 620, and it may be returned tothe ‘0110’ state after processing (as determined, for example, by apacket trailer). Of course, it may be that the circuit-switched path isidentical to the label-switch state path, in which case there is no needto even modify the local route through switch 601 for processing thelabel-switch state. However, if it is necessary to temporarily alterswitch 601, the underlying circuit-switched traffic, if any, can bere-routed or re-sent.

As discussed so far, label switching allows destination oriented routingof packets without a need for the network elements to examine the entiredata packets. New signaling information—the label—is added in the formof optical signal header 210 which is carried in-band within eachwavelength in the multi-wavelength transport environment. This labelswitching normally occurs on a packet-by-packet basis. Typically,however, a large number of packets will be sequentially transportedtowards the same destination. This is especially true for bursty datawhere a large block of data is segmented in many packets for transport.In such cases, it is inefficient for each particular network element tocarefully examine each label and decide on the routing path. Rather, itis more effective to set up a “virtual circuit” from the source to thedestination. Header 210 of each packet will only inform continuation orending of the virtual circuit, referred to as a flow state connection.Such an end-to-end flow state path is established, and the plug-and-playmodules in the network elements will not disrupt such flow stateconnections until disconnection is needed. The disconnection will takeplace if such a sequence of packets have come to an end or anotherpacket of much higher priority requests disruption of this flow stateconnection.

The priority aspect of the present invention is also shown with respectto FIG. 6. The local look-up table has a “priority level” (column 613)which sets forth the priority assigned to the label-switching state.Also, header 210 has appended priority data shown as the number ‘2’(reference numeral 616). Both the fourth and fifth row in the“label-switch state” column 611 of table 610 have a local address of‘0111.’ If an earlier data packet used the entry in the fifth row toestablish, for example, a virtual circuit or flow switching state, andthe now another packet is processed as per the fourth row of column 611,the higher priority data (‘2’ versus ‘4’, with ‘1’ being the highest)has precedent, and the virtual circuit would be terminated.

Detailed Illustrative Embodiments

In order to achieve ultra-low latency IP over WDM label switching,processing of the optical header at each optical switch must be kept toa minimum during the actual transmission of the optical packet. Toachieve this end, a new signaling architecture and packet transmissionprotocol for performing optical WDM label switching is introduced.

The signaling and packet transmission protocols decouple the slow andcomplex IP routing functions from the ultra-fast WDM switching andforwarding functions. This decoupling is achieved via the setup of anend-to-end routing path which needs to be performed very infrequently.To send IP packets from a source to a destination, the following step isexecuted in accordance with the present invention: optical packettransmission, where the arrival of the optical packet triggers the localheader processing which among other things looks up the output port forforwarding the packet on to the next hop based on the optical labelinside the optical header.

Although routing path setup involves invoking the routing function whichis generally a slow and complicated procedure, it is performed prior topacket transmission handling, and hence it is not in the critical paththat determines transmission latency.

Routing Path Setup

During routing path setup, the internal connection table of a WDM packetswitch will be augmented with a label-switch look-up table, and containsthe pertinent packet forwarding information. In particular, in theinterest of achieving ultra-low latency and hardware simplicity, theinventive scheme produces label-switch states that remain constant alongthe flow path. For example, label-switch assignments include thefollowing techniques:

(1) Destination-based flow label assignment—In this scheme thedestination, e.g. a suitable destination IP address prefix, can be usedas the label-switch state in next hop look-up In addition to having noneed to modify the optical header, the same header can be used in theevent of deflection routing.

(2) Route-based flow label assignment—In this scheme the label-switchstate assigned refers to the end-to-end route that is computeddynamically at the label-switch state setup phase. The advantage of thisscheme is that it can be specialized to meet the Quality-of-Servicerequirements for each individual label-switched states.

Switching Conflict Resolution

The present-day lack of a viable optical buffer technology implies thatconventional buffering techniques cannot be used to handle switchingconflicts. As previously described, the invention embodiment utilizesfixed delay implemented by an optical fiber to allow switching to occurduring this time delay, but not to achieve contention resolution aselectrical buffers do in conventional IP routers. To resolve switchingcontentions, in accordance with the present invention, the followingthree methods are used:

(a) Limited wavelength interchange—where a packet is routed through thesame path but at a different wavelength. Since this wavelengthconversion is utilized just to avoid the contention, it is not necessarythat the network elements must possess the capability of converting toany of the entire wavelength channels. Rather, it is sufficient if theycan convert some of the entire wavelength channels. This wavelengthconversion converts both the signaling header and the data payload. Caremust be taken to prevent a packet from undergoing too many wavelengthconversions which will result in poor signal fidelity. A possible policyis to allow only one conversion, which and can easily be enforced byencoding the original wavelength in the optical header. This way anintermediate WDM switch will allow conversion if and only if it iscarried on its original wavelength.

(b) Limited deflection routing—where a packet may be deflected to aneighboring switching node from which it can be forwarded towards itsdestination. Care again must be taken to prevent a packet from beingrepeatedly deflected, thereby causing signal degradation, as well aswasting network bandwidth. A solution scheme is to record a “timestamp”field in the optical header, and allow deflections to proceed if andonly if the recorded timestamp is no older than a maximum limit.

(c) Prioritized packet preemption—where a newly arrived packet maypreempt a currently transmitting packet if the arriving packet has ahigher priority. The objective is to guarantee fairness to all packetsso that eventually a retransmitted packet can be guaranteed delivery. Inthis scheme, each packet again has a timestamp field recorded in itsoptical header, and older packets have higher priority compared to newerpackets. Furthermore a retransmitted packet assumes the timestamp of theoriginal packet. This way, as a packet “ages,” it increases in priority,and will eventually be able to preempt its way towards its destinationif necessary.

It is noted that in all these schemes the optical header can remainconstant as it moves around in the network. This is consistent with thedesire to keep the optical switching hardware fast and simple. It isalso possible to consider combinations of these schemes.

Routing Protocol

For a network the size of the NGI, centralized routing decisions arequite unfeasible, so the approach needs to be generalized to distributeddecision making. Hierarchical addressing and routing are used as in thecase of IP routing. When a new connection is requested, NC&M 220 decideswhether a WDM path is provisioned for this (source, destination) pairwithin the WDM-based network. If it is, the packets are immediately sentout on that (one-hop IP-level) path. If no such path is provisioned,NC&M 220 decides on an initial outbound link for the first WDM networkelement and a wavelength to carry the new traffic. This decision isbased on the rest of the connections in the network at the time the newconnection was requested. NC&M 220 then uses signaling, through anappropriate protocol, to transfer the relevant information to theinitial WDM network element to be placed in the signaling header. Afterthe initial outbound link is determined, the rest of the routingdecisions are taken at the individual networks elements (NE's) accordingto the optical signaling header information. This method ensures thatthe routing tables at each switching node and the signaling headerprocessing requirements are kept relatively small. It also enables thenetwork to scale easily in terms of switching nodes and network users.It is noted, too, that multiple WDM subnetworks can be interconnectedtogether and each subnetwork will have its own NC&M.

When a path is decided upon, within a WDM NE, the optical switches canbe set in that state (i) for the duration of each packet through thenode and then revert back to the default state (called opticallabel-switching), or (ii) for a finite, small amount of time (calledflow switching). The former case performs routing on a regularpacket-by-packet basis. The system resources are dedicated only whenthere is information to be sent and at the conclusion of the packet,these resources are available for assignment to another packet. Thelatter case is used for large volume bursty mode traffic. In this case,the WDM NE only has to read a flow state label from the opticalsignaling header of subsequent packets arriving at the NE to be sure asuch packet is bound for the same destination, without the need toswitch the switching device, and forward the payload through the alreadyexisting connection through the NE as previously established by theoptical label-switching.

The packets are self-routed through the network using the information inthe signaling header of each packet. When a packet arrives at aswitching node, the signaling header is read and either the packet isforwarded immediately through an already existing flow state connectionor a new appropriate outbound port is chosen according to the routingtable. Routing tables in each node exist for each wavelength. If thepacket cannot follow the selected outbound port because of contentionwith another packet (the selected outbound fiber is not free), therouting scheme will try to allocate a different wavelength for the sameoutbound port (and consequently the signal will undergo wavelengthtranslation within the switching node). If no other eligible wavelengthcan be used for the chosen outbound port, a different outbound port maybe chosen from another table, which lists secondary (in terms ofpreference) outbound links.

This routing protocol of the inventive technique is similar to thedeflection routing scheme (recall the Background Section), where thesession is deflected to some other outbound link (in terms ofpreference) if the preferred path cannot be followed. The packet is notallowed to be continuously deflected. In traditional routing protocols,a hop count is used to block a session after a specified number of hops.In the new scheme, in case no header regeneration is allowed at theswitching nodes, then the hop count technique cannot be used.Alternatively, the optical signaling header characteristics (i.e., thesignaling header's signaling to noise ratio) can be looked upon todecide whether a packet should be dropped.

IP Routing Algorithm in WDM Layer

The technique used by NC&M 220 to determine the routing tables is basedupon shortest path algorithms that route the packets from source todestination over the path of least cost. Specific cost criteria on eachroute, such as length, capacity utilization, hop count, or averagepacket delay can be used for different networks. The objective of therouting function is to have good performance (for example in terms oflow average delay through the network) while maintaining highthroughput. Minimum cost spanning trees are generated having a differentnode as a root at each time, and the information obtained by these treescan then be used to set-up the routing tables at each switching node. Ifdeflection routing as outlined above is implemented, the k-shortest pathapproach can be used to exploit the multiplicity of potential routingpaths. This technique finds more than one shortest path, with the pathsranked in order of cost. This information can be inputted into theswitching node routing tables, so that the outbound link correspondingto the minimum cost path is considered first, and the linkscorresponding to larger cost paths are inputted in secondary routingtables that are used to implement deflection routing.

Description of Plug-and-Play Modules

The present invention is based upon two types of Plug-and-Play modulesto be attached to the WDM network elements. Introduction of thesePlug-and-Play modules add optical label switching capability to theexisting circuit-switched network elements.

In FIG. 3, both header encoder 321 and header remover 322 were shown inhigh-level block diagram form; FIGS. 7 and 8 show, respectively, a moredetailed schematic for both encoder 321 and remover 322.

In FIG. 7, IP packets or datagrams are processed in microprocessor 710which generates each optical signaling header 210 for label switching.Optical signaling header 210 and the original IP packet 211 are emittedfrom microprocessor 710 at baseband. Signaling header 210 is mixed in RFmixer 720 utilizing local oscillator 730. Both the mixed header frommixer 720 and the original packet 211 are combined in combiner 740 and,in turn, the output of combiner 740 is encoded to an optical wavelengthchannel via optical modulator 760 having laser 750 as a source ofmodulation.

In FIG. 8, the optical channel dropping out of a network element isdetected by photodetector 810 and is electrically amplified by amplifier820. Normally, both photodetector 810 and the amplifier 820 have afrequency response covering only the data payload but not the opticalsignaling header RF carrier frequency provided by local oscillator 730.Low-pass-filter 830 further filters out any residual RF carriers. Theoutput of filter 830 is essentially the original IP packet sent out bythe originating IP router from the originating network element which hasbeen transported through the network and is received by another IProuter at another network element.

Block diagram 900 of FIG. 9 depicts the elements for the detectionprocess effected by Plug-and-Play module 410 of FIG. 4 to convertoptical signal 901, which carries both label-switching signaling header210 and the data payload 211, into baseband electrical signaling header902. Initially, optical signal 901 is detected by photodetector 910; theoutput of photodetector 910 is amplified by amplifier 920 and filteredby high-pass filter 930 to retain only the high frequency componentswhich carry optical signaling header 210. RF splitter 940 provides asignal to local oscillator 950, which includes feedback locking. Thesignal from local oscillator 950 and the signal from splitter 940 aremixed in mixer 960, that is, the high frequency carrier is subtractedfrom the output of filter 920 to leave only the information onlabel-switching signaling header 210. In this process, local oscillator950 with feedback locking is utilized to produce the local oscillationwith the exact frequency, phase, and amplitude, so that the highfrequency component is nulled during the mixing of this local oscillatorsignal and the label-switching signaling header with a high-frequencycarrier. Low-pass filter 970, which is coupled to the output of mixer960, delivers baseband signaling header 210 as electrical output signal902.

The circuit diagram of FIG. 10 shows an example of a more detailedembodiment of FIG. 4. In FIG. 10, each header detector 1010, 1020, . . ., 1050, . . . , or 1080 processes information from each wavelengthcomposing the optical inputs arriving on paths 1001, 1002, 1003, and1004 as processed by demultiplexers 1005, 1006, 1007, and 1008,respectively; each header detector is exemplified by the circuit 900 ofFIG. 9. The processed information is grouped for each wavelength. Thus,for example, fast memory 1021 receives as inputs, for a givenwavelength, the signals appearing on lead 1011 from header detector thesignals appearing on lead 1012 from header detector 1030, the signalsappearing on lead 1013 from header detector 1050, and the signalsappearing on lead 1014 from header detector 1070. Each fast memory1021-1024, such as a content-addressable memory, serves as an input to acorresponding label switch controller 1031-1034. Each label switchcontroller 1031-1034 also receives circuit-switched control signals fromnetwork element switch controller 420 of FIG. 4. Each label switchcontroller intelligently chooses between the circuit switched control asprovided by controller 420 and the label switched information suppliedby its corresponding fast memory to provide appropriate control signalsthe switching device 430 of FIG. 4.

Flow diagram 1100 of FIG. 11 is representative of the processingeffected by each label-switch controller 1031-1034. Using label-switchcontroller 1031 as exemplary, inputs from circuit-switched controller420 and inputs from fast memory 1021 are monitored, as carried out byprocessing block 1110. If no inputs are received from fast memory 1021,then incoming packets are circuit-switched via circuit-switchedcontroller 420. Decision block 1120 is used to determine if there areany inputs from fast memory 1021. If there are inputs, then processingblock 1130 is invoked so that label-switch controller 1031 can determinefrom the fast memory inputs the required state of switching device 430.Then processing block 1160 is invoked to transmit control signals fromlabel-switch controller 1031 to control switching device 430. If thereare no fast memory inputs, then the decision block 1140 is invoked todetermine if there are any inputs from circuit-switched controller 1140.If there are inputs from circuit-switched controller 420, thenprocessing by block 1150 is carried out so that label-switch controller1031 determines from the inputs of circuit-switched controller 420 therequired state of switching device 430. Processing block 1160 is againinvoked by the results of processing block 1150. If there are no presentinputs from circuit-switched controller 1140 or upon completion ofprocession block 1160, control is returned to processing block 1110.

By way of reiteration, optical label-switching flexibly handles alltypes of traffic: high volume burst, low volume burst, and circuitswitched traffic. This occurs by interworking of two-layer protocols ofthe label-switched network control. Thus, the distributed switchingcontrol rapidly senses signaling headers and routes packets toappropriate destinations. When a long stream of packets reach thenetwork element with the same destination, the distributed switchingcontrol establishes a flow switching connection and the entire stream ofthe packets are forwarded through the newly established connections.

A label switching method scales graciously with the number ofwavelengths and the number of nodes. This results from the fact that thedistributed nodes process multi-wavelength signaling information inparallel and that these nodes incorporate predicted switching delay inthe form of fiber delay line. Moreover, the label switching utilizespath deflection and wavelength conversion for contention resolution.

Optical Header Processing

The foregoing description focused on optical header processing at alevel commensurate with the description of the overall NGI systemconfigured with the overlaid Plug-and-Play modules. Discussion of headerprocessing at a more detailed level is now appropriate so as toexemplify how low-latency can be achieved at the circuit-detail level.

To this end, opto-electrical circuitry 1200 of FIG. 12, which is a moredetailed block diagram elucidating certain aspects of prior figures,especially FIGS. 9 and 10, is considered. By way of a heuristicoverview, the processing carried out by the opto-electrical circuitry1200 is such that a header signal (e.g., 155 Mb/s on a microwavecarrier) is frequency-division multiplexed with a baseband data payload(e.g., 2.5 Gb/s). The header signal is processed by a single-sideband(SSB) modulator, so only the upper sideband representation of the headersignal is present in the frequency-division multiplexed signal.Moreover, the technique effected by circuitry 1200 is one of labelreplacement wherein the original header signal at the given carrierfrequency is first removed in the optical domain, and then a new headersignal is inserted at the same carrier frequency in the optical domain.A notch filter is used to remove the original header signal; the notchfilter is realized, for example, by the reflective part of a Fabry-Perotfilter.

In particular, circuitry 1200 has as its input the optical signal atoptical wavelength λ₁ on path 1001 as received and processed by demux1005, both of which are re-drawn from FIG. 10. Circuitry 1200 iscomposed of: a lower path to process optical signal 1201 emanating fromdemux 1005; and an upper path to process optical signal 1202 emanatingfrom demux 1005. The lower path derives the label, conveyed by theincoming SSB header in optical signal 1001, to control optical switch1203; switch 1203 is a multi-component element encompassing componentsalready described, including fast memory 1021 and label switchcontroller 1031 of FIG. 10 as well as switching device 430 of FIG. 4.The upper path is used to delete the old header signal, including thelabel, at the sub-carrier frequency and then insert a new header label,in a manner to be described below after the lower path is firstdescribed.

The lower path is an illustrative embodiment of header detector 1010originally shown in high-level block diagram form in FIG. 10. Inparticular, header detector 1010 includes, in cascade: (a)opto-electrical converter 1210 (e.g., a photodetector) for producingelectrical output signal 1211; (b) multiplier 1215 to convert electricalsignal 1211 to intermediate frequency signal 1217—to accomplish this,multiplier 1215 is coupled to local oscillator 1218 which provides asinusoid 1216 at a frequency to down-convert the incoming sub-carrierconveying the header label, designated for discussion purposes as f_(c),to an intermediate frequency f_(I); (c) intermediate frequency bandpassfilter 1220 having signal 1217 as its input; (e) demodulator 1225 toconvert the intermediate frequency to baseband; (e) detector 1230responsive to demodulator 1225; and (f) read circuit 1235 which outputssignal on lead 1011 of FIG. 10. Elements 1211, 1215, 1216, 1217, 1220,1225, and 1230 can all be replaced by a simple envelope detector if thesubcarrier header was transmitted using an incoherent modulator such asASK (amplitude-shift keying). It is not always required to use acoherent demodulator as shown in FIG. 12. (In fact, FIG. 13 will depictthe case for an incoherent modulation).

The operation of header detector 1010 of FIG. 12 is as follows. It isassumed that the second type of ‘Plug-and-Play’ module of FIG. 4 injectsa 2.5 Gbps IP data packet (e.g., with QPSK/QAM modulat ion) which issub-carrier multiplexed with a 155 Mbps single-sideband header packet(e.g., with SSB modulation) at the modulation frequency f_(c); asbefore, the header precedes the data payload in time and both arecarried by the optical wavelength λ₁. In each network node that receivesthe combined header and payload at wavelength λ₁, the sub-carrier headerat f_(c) is multiplied by multiplier 1215, is band-pass filtered byintermediate filter 1220, and is demodulated to baseband by demodulator1225. Then, the demodulated baseband data burst is detected by detector1230 (e.g., a 155 Mbps burst-mode receiver), and read by circuit 1293(e.g., a microprocessor).

This foregoing operational description has focused only on the detectionof the optical header to control the routing path through switch 1203.As alluded to in the Background Section, header replacement is nowconsidered important to present-day NGI technology so as to accomplishhigh-throughput operation in a packet switched network in which datapaths change due to, for example, link outages and variable trafficpatterns. Moreover, header replacement is useful to maintain protocolcompatibility. The upper path components of FIG. 12 that have heretoforenot been described play a central role in header replacement. Actually,the notion of header replacement has a broader connotation in that theheader may be composed of various fields, such as a “label” field and a“time-to-leave” field. The description to this point has used the headerand label interchangeably; however, it is now clear that the header mayactually have a plurality of fields, and as such any or all may bereplaced at any node.

Now continuing with the description of FIG. 12, the upper processingpath which processes the optical signal on path 1202 includes: (a)circulator 1240; (b) Fabry-Perot (FFP) filter 1245, coupled tocirculator 1240 via path 1241, with filter 1245 being arranged so thatone notch in its free spectral range (FSR) falls at f_(c); and (c)attenuator 1250 coupled to the reflective port (R) of FFP 1245. Anexemplary FFP 1245 is available from The Micron Optics, Inc. as modelNo. FFP-TF (“Fiber Fabry-Perot Tunable Filter”). The combination ofthese latter three elements, shown by reference numeral 1251, produces anotch filter centered at f_(c) which removes the SSB header signalpropagating with f_(c) as its center frequency, as shown pictorially bythe spectra in the upper portion of FIG. 12. As illustrated, spectrum1242 of signal 1202 includes both a baseband data spectrum and theheader signal spectrum centered at f_(c). After processing by notchfilter 1251, spectrum 1243 obtains wherein only the baseband dataspectrum remains.

The output of notch filer 1251, appearing on path 1244 of circulator1240, serves as one input to Mach-Zender modulator (MZM) 1270. Two otherinputs to MZM 1270 are provided, namely, via path 1271 emanating frommultiplier 1290 and via path 1272 emanating from phase shift device1295. As discussed in the next paragraph, the signal appearing on lead1271 is the new header signal which is double-sideband in nature. Thesignal on path 1272 is phase-shifted by {fraction (π/2)} relative to thesignal on path 1271. MZM 1270 produces at its output the upper-sidebandversion of the signal appearing on path 1271, that is, the new headersignal. The single-sideband processing effected by MZM 1270 is describedin detail in the paper entitled “Overcoming Chromatic-Dispersion Effectsin Fiber-Wireless Systems Incorporating External Modulators”, authoredby Graham H. Smith et al., as published in the IEEE Transactions onMicrowave Theory and Techniques, Vol. 45, No. 8, August 1997, pages1410-1415, which is incorporated herein by reference. Moreover, besidesconverting the new header signal to an optical single-sideband signal(OSSB), MZM 1270 also adds this OSSB signal to the incoming opticalbaseband signal on path 1244 to produce the desiredfrequency-multiplexed signal of baseband plus SSB header on output path1273 from MZM 1270.

The new header signal delivered by path 1271 is derived as follows.Write circuit 1275 is responsible for providing data representative of anew header signal, such as a new label represented in binary. The headersignal that arrives at the input to demux 1005 is referred to as theactive header signal. The replacement header signal is called the newheader signal. Write circuit 1275 has as its input the output of readdevice 1235, so write circuit 1275 can reference or use information fromthe active header signal to derive the new header signal, if necessary.The new header signal, as provided at the output of write circuit 1275,is delivered to pulse generator 1280, which performs the operation ofconverting the new header signal data to, as exemplary, a 155 Mb/ssignal on a microwave carrier. The signal from generator 1280 isfiltered by low-pass filter 1285 to remove spurious high-frequencyenergy. Then the signal from filter 1285 is delivered to modulator 1290;modulator 1290 also has as a sinusoidal input at frequency f_(c)provided by local oscillator 1218. The output of modulator 1290, whichappears on path 1271, is the new header signal centered at a frequencyof the local oscillator, namely f_(c); also, the output of modulator1290 serves as the only input to phase-shift device 1295.

MZM 1270 produces a spectrum that includes both the original basebanddata spectrum as well as the spectrum of the new header signal at f_(c).This is shown in frequency domain visualization 1274 in the topright-hand corner of FIG. 12, which is counterpart of the visualizationin the top left-hand corner.

The new optical signal on path 1273 is switched via optical switch 1203,as controlled by the active or original incoming header signal, undercontrol of the label on lead 1011.

It is noted that, in terms of presently available components, theprocessing time of the header removal and insertion technique shouldtake less than 30 ns. On the other hand, if it is assumed that there are15 bits in each packet header signal, then the time to read 15 bits,write 15 bits, and add 10 preamble bits can take about 260 ns for a 155Mbps burst. Therefore, allowing for some variations, each header signalis about 300 ns. This means that it may be necessary to insert a delayline in the main optical path between circulator 1240 and MZM 1270 of300 ns, so the length of delay line would be around 60 meters. To saveprocessing time, the data rate of the subcarrier header can be increasedto, for example, 622 Mb/s or higher, depending upon the future networkenvironment.

Another Illustrative Embodiment of a Header Removal and InsertionTechnique

The circuit arrangement of FIG. 12 is realized using the so-calledreflective port of FFP 1245. FFP 1245 also has a transmission port whichmay be utilized wherein the characteristics of the optical signalemanating from the transmission port are the converse of the opticalsignal from the reflective port. So whereas the reflective port providesan attenuation notch at f_(c), the transmission port attenuatesfrequencies relative to f_(c), so that only frequencies in the vicinityof f_(c) are passed by the transmission port. An alternative tocircuitry 1200 of FIG. 12 is shown by circuitry 1300 of FIG. 13. Themain difference between FIGS. 12 and FIG. 13 is the manner in which thelower processing path now derives its input signal via path 1301 (ascompared to input signal on path 1201 of FIG. 12).

In particular, FFP 1325 now has a transmission (T) port in addition tothe reflective (R) port. The output from transmission port, on path1301, now serves as the input to opto-electrical converter 1210. Becausethe signal on path 1301 conveys only frequencies centered about f_(c),that is, the baseband data information has been attenuated by FFP notchfilter 1345, and can be processed directly by detector 1230 via LPF1320. The remainder of circuitry 1300 is essentially the same ascircuitry 1200 of FIG. 12.

Optical Technology

Optical technologies span a number of important aspects realizing thepresent invention. These include optical header technology, opticalmultiplexing technology, optical switching technology, and wavelengthconversion technology.

(a) Optical Header Technology

Optical header technology includes optical header encoding and opticalheader removal as discussed with respect to FIGS. 3 and 4. In effect,optical header 210 serves as a signaling messenger to the networkelements informing the network elements of the destination, the source,and the length of the packet. Header 210 is displaced in time comparedto the actual data payload. This allows the data payload to have anydata rates/protocols or formats.

(b) Optical Multiplexing Technology

Optical multiplexing may illustratively be implemented using the knownsilica arrayed waveguide grating structure. This waveguide gratingstructure has a number of unique advantages including: low cost,scalability, low loss, uniformity, and compactness.

(c) Optical Switching Technology

Fast optical switches are essential to achieving packet routing withoutrequiring excessively long fiber delay as a buffer.

Micromachined Electro Mechanical Switches offer the best combination ofthe desirable characteristics: scalability, low loss, polarizationinsensitivity, fast switching, and robust operation. Recently reportedresult on the MEM based Optical Add-Drop Switch achieved 9 microsecondswitching time.

(d) Wavelength Conversion Technology

Wavelength conversion resolves packet contention without requiring pathdeflection or packet buffering. Both path deflection and packetbuffering cast the danger of skewing the sequences of a series ofpackets. In addition, the packet buffering is limited in duration aswell as in capacity, and often requires non-transparent methods.Wavelength conversion, on the other hand, resolves the blocking bytransmitting at an alternate wavelength through the same path, resultingin the identical delay. Illustratively, a WSXC with a limited wavelengthconversion capability is deployed.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A method for replacing a header and for routingan optical signal propagating at a given optical wavelength, the opticalsignal including both the header and a data payload, the header beingconveyed by a single-sideband signal occupying a given frequency bandabove the data payload, the method comprising the steps of detecting theheader to produce a switch control signal to route the optical signal,optically filtering the optical signal with a reflective part of a notchfilter to delete the header and recover the data payload, and insertinga new single-sideband header at the given frequency band into theoptical signal in place of the deleted header.
 2. The method as recitedin claim 1 wherein the notch filter is a Fabry-Perot filter having afree-spectral range centered on the given frequency band.
 3. The methodrecited in claim 2 wherein the step of detecting includes the steps ofopto-electrically converting the optical signal to detect the header,demodulating the detected header to produce header information, andreading the header information to produce the switch control signal. 4.The method as recited in claim 1 wherein the step of inserting includesthe step of single-sideband modulating the new header with a carrier inthe given frequency band.
 5. The method as recited in claim 1 whereinthe step of inserting includes the step of single-sideband modulatingthe new header and the data payload with a Mach-Zender modulator.
 6. Themethod as recited in claim 1 further including an add-drop opticalswitch controlled by the switch control signal and wherein the step ofinserting further includes the step of supplying the new single-sidebandheader and data payload to the add-drop optical switch.
 7. The method asrecited in claim 1 wherein the step of inserting the new header includesthe step of referencing a local routing table for routes through theoptical switch.
 8. The method as recited in claim 1 wherein the step ofdetecting includes the steps of opto-electrically converting the opticalsignal to detect the header, and processing the header to produce theswitch control signal to route the optical signal.
 9. The method asrecited in claim 8 wherein the step of processing includes the step ofdemodulating the detected header to produce header information.
 10. Themethod as recited in claim 9 wherein the step of processing furtherincludes the step of reading the header information to produce theswitch control signal.
 11. The method as recited in claim 8 wherein thestep of processing includes the steps of generating an intermediatefrequency (IF) electrical signal from the detected header, demodulatingthe IF electrical signal to produce a demodulated signal, detecting thedemodulated signal to produce header information, and reading the headerinformation to produce the switch control signal.
 12. A method forreplacing a header and for routing an optical signal propagating at agiven optical wavelength, the optical signal including both the headerand a data payload, the header being conveyed by a single-sidebandsignal occupying a given frequency band above the data payload, themethod comprising the steps of opto-electrically converting the headerto produce header information, reading the header information to producea switch control signal to route the optical signal, optically filteringthe optical signal with a reflective part of the notch filter to deletethe header and recover the data payload, and inserting a newsingle-sideband header at the given frequency band into the opticalsignal in place of the deleted header with a Mach-Zender modulator. 13.A system for replacing a header and for routing an optical signalpropagating at a given optical wavelength, the optical signal includingboth the header and a data payload, the header being conveyed by asingle-sideband signal occupying a given frequency band above the datapayload, the system comprising means for detecting the header to producea switch control signal to route the optical signal, an optical notchfilter for filtering the optical signal with a reflective part of thenotch filter to delete the header and recover the data payload, andmeans, coupled to the notch filter, for inserting a new single-sidebandheader at the given frequency band into the optical signal in place ofthe deleted header.
 14. The system as recited in claim 13 wherein thenotch filter is a Fabry-Perot filter having a free-spectral rangecentered on the given frequency band.
 15. The system as recited in claim14 wherein the means for detecting includes an opto-electrical converterfor converting the optical header to a detected header, a generator forgenerating an intermediate frequency (IF) electrical signal from thedetected header, a demodulator for demodulating the IF electrical signalto produce a demodulated signal, a detector for detecting thedemodulated signal to produce header information, and a reader forreading the header information to produce the switch control signal. 16.The system as recited in claim 13 wherein means for inserting includes asingle-sideband modulator for modulating the new header with a carrierin the given frequency band.
 17. The system as recited in claim 13wherein the means for inserting includes a Mach-Zender single-sidebandmodulator for modulating the new header and the data payload.
 18. Thesystem as recited in claim 13 wherein the means for detecting includesan opto-electrical converter for converting the optical header to adetected header, and a processor, coupled to the opto-electricalconverter, for processing the header to produce the switch controlsignal to route the optical signal.
 19. The system as recited in claim18 wherein the processor includes a demodulator for demodulating thedetected header to produce header information.
 20. The system as recitedin claim 19 wherein the processor further includes a reader for readingthe header information to produce the switch control signal.
 21. Thesystem as recited in claim 18 wherein the processor includes a generatorfor generating an intermediate frequency (IF) electrical signal fromdetected header, a demodulator for demodulating the IF electrical signalto produce a demodulated signal, a detector for detecting thedemodulated signal to produce header information, and a reader forreading the header information to produce the switch control signal. 22.The system as recited in claim 13 further including an add-drop opticalswitch controlled by the switch control signal and wherein the means forinserting further includes means for supplying the new single-sidebandheader and data payload to the add-drop optical switch.